Neutron disappearance and the LZ nuclear recoil event

RESUMO

The LZ experiment reported a nuclear recoil with anomalous energy $E_R=248$ keV, around the typical neutron motion in xenon. This motivates interpreting the event as the disappearance of a bound neutron, either through spontaneous decay or a dark-matter-induced reaction. If the energy released is large, the predicted recoil spectrum is broad around the observed value; only $\sim10\%$ of xenon events avoid extra nuclear activity; the disappearance rate is target-independent and mildly constrained by oxygen experiments. In the special case of near-threshold DM-induced neutron disappearance, it can be open in xenon and closed in oxygen, the quiet fraction rises to $\sim30\%$, the recoil spectrum turns into lines.